welcome to the forums!
specific sound qestions like this are always hard to answer, because so much of it is subjective..
so, i'll just go over the different options
a little preface: i am not a 'uber-audiophile' i like clean, loud music, but am not to picky about the bitrate of my digital media as long as it is above 128bps, and really cant tell a difference between a 16 bit audio stream and 24 bit stream...
for audio processing, you could use a audio program like audiomulch or console, and then send it out to the amps.
or you can just use a optical cable to a processor like the bitone.
for speakers, my 3 way active setup(2way+sub) is louder than most fullrange speakers-- because each speaker is getting only the frequencies that it can play easily, and there is no longer any passive crossovers sucking power away from the audio.
rear speakers are personal preferance, but i don't use them in my families ranger, and have disconnected them in my own car--i think that the closer the rear speakers are to your seat, the less you need them, and the more they hurt the imaging of the front speakers